Jamieson, Fausset and Brown on Zechariah 14:6

6. light ... not ... clear ... dark--Jerome, Chaldee, Syriac, and Septuagint translate, "There shall not be light, but cold and ice"; that is, a day full of horror (Am 5:18). But the Hebrew for "clear" does not mean "cold," but "precious," "splendid" (compare Job 31:26). Calvin translates, "The light shall not be clear, but dark" (literally, "condensation," that is, thick mist); like a dark day in which you can hardly distinguish between day and night. English Version accords with Zec 14:7: "There shall not be altogether light nor altogether darkness," but an intermediate condition in which sorrows shall be mingled with joys.